The four-volume set LNCS 9296-9299 constitutes the refereed proceedings of the 15th IFIP TC13 International Conference on Human-Computer Interaction, INTERACT 2015, held in Bamberg, Germany, in September 2015. The 41 papers included in the first volume are organized in topical sections on accessibility; accessible interfaces for blind people; accessible interfaces for older adults; affective HCI and emotions and motivational aspects; alternative input; alternative input devices for people with disabilities; interfaces for cognitive support; brain-computer interaction; cognitive factors.